1. Technological Field
The present invention relates to spectral cameras, to methods of configuring such cameras, and to methods of operating such cameras.
2. Description of the Related Technology
Spectral cameras are known and some are referred to as multi spectral or hyperspectral imaging systems.
Hyperspectral imaging refers to the imaging technique of collecting and processing information from across the electromagnetic spectrum. Whereas the human eye only can see visible light, a hyperspectral imaging system can see visible light as well as from the ultraviolet to infrared. Hyperspectral sensors thus look at objects using a larger portion of the electromagnetic spectrum, as has been described at: http://en.wikipedia.org/wiki/Hyperspectral_imaging.
Certain objects leave unique “fingerprints” across this portion of the electromagnetic spectrum. These “fingerprints” are known as spectral signatures and enable identification of the materials that make up a scanned object. The hyperspectral capabilities of such an imaging system enable to recognize different types of objects, all of which may appear as the same color to the human eye.
Whereas multispectral imaging deals with several images at discrete and somewhat narrow bands, hyperspectral imaging deals with imaging narrow spectral bands over a contiguous spectral range. It can produce the spectra for all pixels in the scene. While a sensor with 20 discrete bands covering the VIS, NIR, SWIR, MWIR, and LWIR would be considered multispectral, another sensor with also 20 bands would be considered hyperspectral when it covers the range from 500 to 700 nm with 20 10-nm wide bands.
Hyperspectral sensors collect information as a set of “images.” Each image represents a range of the electromagnetic spectrum and is also known as a spectral band. These images each have two spatial dimensions and if images of a series of different spectral bands are effectively stacked to form a cube, then the third dimension can be a spectral dimension. Such a three dimensional hyperspectral cube is a useful representation for further image processing and analysis. The precision of these sensors is typically measured in spectral resolution, which is the width of each band of the spectrum that is captured. If the scanner picks up on a large number of fairly narrow frequency bands, it is possible to identify objects even if said objects are only captured in a handful of pixels. However, spatial resolution is a factor in addition to spectral resolution. If the pixels are too large, then multiple objects are captured in the same pixel and become difficult to identify. If the pixels are too small, then the energy captured by each sensor-segment is low, and the decreased signal-to-noise ratio reduces the reliability of measured features.
Current hyperspectral cameras produce a hyperspectral datacube or image cube, consisting of a stack of 2D images in the x-y plane of the scene in which each image of the stack contains information from a different frequency or spectral band. The spectral range that is captured is not limited to visual light, but can also span Infra Red (IR) and/or Ultra Violet (UV). The 3D Image Cube is captured by a hyperspectral imager, using an array of sensors that is inherently a 2D sensor. Therefore some form of scanning can be used, so that the cube is assembled over a number of frame periods.
Line scanners or pushbroom systems thus capture a single line of the 2D scene in all spectral bands in parallel. To cover all spatial pixels of the scene, this type of system then scans different lines over time, for example by relative movement of the scanner and the scene. Starers or staring systems capture the complete scene in a single spectral band at a time with a 2D array of sensors and scan over different spectral bands in order to produce the 3D hyperspectral image cube.
It is known from the article entitled Design and fabrication of a low-cost, multispectral imaging system by Scott A. Mathews to provide optical duplication onto an array of sensors. Cross talk between image copies is limited by a physical barrier which covers some of the sensor elements. This was available at: http://faculty.cua.edu/mathewss/journals/Appl%20Opt%20V47%20N28%202008.pdf.
Another known device using such optical duplication is a “miniature snapshot multispectral imager” by Infotonics technology center. Again this avoids cross talk by having walls between the image copies on the sensor array.